Comets

World of Earth Science
COPYRIGHT 2003 The Gale Group, Inc.

Comets

Comets are objects—relatively small compared to planets—that are composed of dust and ices of various compounds. Comets orbit the Sun in elongated elliptical (eccentric, elongated circle) or parabolic orbits. Accordingly, these objects spend the majority of time in the outer regions of the solar system , in some cases well beyond the orbits of Neptune and Pluto. Short-period comets are those with less exaggerated elliptical orbits that carry them out only as far as the region of space between the orbits of Jupiter and Neptune. Comets make periodic, brief, but sometimes-spectacular transits through the inner solar system as they approach the Sun. Comet orbits may be prograde, in the same direction as the planets, or retrograde, in the opposite direction. With the aid of a telescope , a comet is usually visible from Earth.

The term "comet" derives from the Greek aster kmetes (translated literally as "hairy" or long-haired star)—a reference to a sometimes-visible comet tail. If a comet's path takes it close enough to the Sun, the heating causes melting and emission of gases (out gassing) and dust that are then swept behind the comet's orbital path (away from the Sun) by the solar wind to form the characteristic tail.

Fascination with objects in the night sky dates to the dawn of human civilization. Etchings on clay tablets unearthed in the ancient city of Babylon dating to at least 3000 b.c. and rock carvings found in prehistoric sites in Scotland dating to 2000 b.c. depict unexplained astronomical phenomena that may have been comets. Until the Arabic astronomers of the eleventh century, the Chinese were by far the most astute skywatchers in the ancient and medieval world. By 400 b.c., their intricate cometary classification system included sketches of 29 comet forms, each associated with a past event and predicting a future one. Comet type 9 was named Pu-Hui, meaning "Calamity in the state, many deaths." In fact, until the seventeenth century when English Astronomer Edmund Halley (1656–1742) predicted the return of a the comet in 1758 (thereafter known as Halley's comet) based, in part, upon calculations derived from English physicist and mathematician Sir Isaac Newton's (1642–1727) work, comets were widely viewed with superstition, as omens portending human disasters and terrestrial catastrophes.

Recorded observation of comets is evident in the records of the Ancient Chinese culture who termed comets "guest stars," a general term also applied to other apparent temporary solar system transients that were later, of course, identified to be much more distant stellar novae. Chinese records clearly indicate the transit of a guest star in approximately 240 b.c. that we now identify as Halley's comet.

In accord with Anasazi Native American accounts, Chinese astronomers also noted the difference in what is now regarded as comets and the 1054 supernova explosion in the Taurus constellation (i.e., a region of the sky associated with the Taurus constellation) that created the Crab Nebula.

Of all the Classical Greek and Roman theories on comets, the most influential, though entirely incorrect, was that of the Greek philosopher Aristotle (384–322 b.c.). His geocentric view of the solar system put Earth at the center circled by the Sun, Moon , and visible planets. Stars were stationary and the bodies existed on celestial spheres. Aristotle argued that comets were fires in the dry, sublunar "fiery sphere," a combustible atmosphere "exhaled" from Earth, which accumulated between Earth and the Moon. Comets were therefore considered terrestrial—originating from Earth, rather than celestial—heavenly bodies. Moreover, they were seen as a portent of future events controlled by the gods.

Aristotle's writings formed the basis of later Greek-Alexandrian astronomer Ptolemy's (a.d. 87–150) model of the universe that became strongly supported by the Christian church in Western Europe . Because the Ptolemy's model provided accurate results with regard to celestial prediction, it was the most influential astronomical model until the acceptance of the Sun-centered (heliocentric) model put forth by Polish astronomer Nicolaus Copernicus (1473–1543).

In conjunction with the German astronomer and mathematician Johannes Kepler (1571–1630), Danish astronomer Tycho Brahe's (1546–1601) observation of the "Great Comet" of 1577 provided evidence that the comet was at least four times further away from Earth than the Moon—a crushing refutation of Aristotle's sublunar positioning.

The study of the Great Comet by Brahe and his contemporaries was the turning point for astronomical science. Throughout the seventeenth and eighteenth centuries, mathematicians and astronomers refined conflicting ideas on the origin, formation, movement, shape of orbit, and meaning of comets. Polish-born scientist Johannes Hevelius (1611–1687), who suggested comets move on a parabola (U-shape) around the Sun; and English scientist Robert Hooke (1635–1703), independent of Newton, introduced the theory of universal gravitational influence based, in part on the periodic behavior of comets. Newton, however, developed an astounding mathematical model for the parabolic motion of comets, published in his seminal and influential 1687 book, Philosophiae Naturalis Principia Mathematica (Mathematical principles of natural philosophy). Until English naturalist Charles Darwin's (1809–1882) writings on evolution and German-American physicist Albert Einstein's (1879–1955) twentieth century writings on relativity theory , Principia remained the single most influential scientific work in the history of science."

By the end of the eighteenth century, comets were understood to be astronomical bodies, the movement of which could be calculated using Newton's laws of planetary motion.

The comet Biela, with a periodic orbit of 6.75 years, split in two during its 1846 appearance. Twin comets reappeared in 1852—but then failed to appear for its next pass. The disappearance fostered scientific speculation regarding comet impacts and their relationships to meteor showers. When Biela's twin offspring should have returned, the meteor shower predicted by some astronomers did indeed appear, strengthening the connection between meteors and dying comets.

The first observation of a comet through a telescope was made in 1618. Until the twentieth century, comets were discovered and observed with the naked eye or through telescopes. Today, most new discoveries are made from photographs of our galaxy and electronic detectors, although many discoveries are still made by amateur astronomers with a passion for careful observation.

The long focal-length refracting telescope, the primary astronomical observation tool of the 1800s, worked well for viewing bright objects but did not collect sufficient light to allow detailed astronomical photography. In 1858, an English artist named Usherwood used a short focal-length lens to produce the first photograph of a comet. In 1864, by using a spectroscope, an instrument that separates the wavelengths of light into spectral bands, Italian astronomer Giovanni Donati (1826–1873) first identified a chemical component in a comet's atmosphere. The first cometary spectrogram (spectral photograph) was taken by amateur astronomer William Huggins of London in 1881.

The early twentieth century saw the development of short focal-length spectrographs that, by the 1950s, allowed identification of several different chemical components in a comet's tail. Infrared spectrography was introduced in the 1960s and, in 1983, the Infrared Astronomy Satellite (IRAS) gathered information on cometary dust particles unattainable from ground-based technology. Observations of comets are now also made by radio wave detection and ultraviolet spectrography.

Spectroscopic evidence indicates that most comets contain a solid nucleus (core) surrounded by a gigantic, glowing mass (coma). Together, the nucleus and coma comprise the comet head. It should be noted that although the tail (when apparent) seems dense with dust and gas, it is still a vacuum that is far less dense than the interplanetary space near the earth (e.g., between the earth and Moon).

Perhaps among the most primitive bodies in the solar system, comets are probably debris from the formation of our Sun and planets some 4.5 billion years ago. One hypothesis

concerning their origin involves the Oort cloud—named for Dutch astronomer Jan Van Oort—a dense shell of debris (dense by interstellar standards) at the frigid, outer edge of the solar system (i.e., the distance at which our Sun's gravitational pull is so weak that beyond this point other stellar bodies exert a greater net attraction). Occasionally, disruptive gravitational forces (perturbations) hurl a piece of debris from the cloud into the gravitational pull of one of the large planets, (e.g. Jupiter or Saturn) that then pull the comet into an elliptical orbit around the Sun. The Kuiper belt, associated with Jupiter's gravitational pull, is more likely the source of the well-known comets, including Halley's comet. Regardless, evidence indicates that comets formed from solar system formation debris.

Short lived comets have orbital durations of less than 200 years. Long-period, having enormous elliptical, nearly parabolic orbital durations of more than 200 years, often traveling far beyond the outer planets. Of the 710 individual comets recorded from 1680 to mid-1992, 121 were short-period comets and 589 were classified as long-period comets.

Two major theories on the composition of the nucleus have developed over time. The "flying sandbank" model, first proposed by Richard Proctor in the mid-1800s and again in the mid-1900s by Raymond Lyttleton, conjectured swarms of tiny solid particles bound together by mutual gravitational attraction. In 1950, Fred Whipple introduced the "icy-conglomerate" model, which described a comet as a solid nucleus of meteoric rock particles and dust frozen in ice . Observations of Halley's comet by spacecraft in 1986 strongly support this model.

Evidence to date indicates that within the comet head or nucleus, rocks and dust are held together with ices from water , methane, ammonia, and carbon monoxide, as well as other ices containing carbon and sulphur. The 1986 studies of Halley's comet revealed the nucleus to be peanut or potato-shaped, 9 mi (15 km) long, and 5.5 mi (8 km) wide. However, visual observation beneath the comet's dark, solid surface proved impossible.

The nuclei of comets are among the smallest bodies in the Solar system, too small, in fact, for observation even through a telescope. As they approach the Sun, however, they produce one of the largest, most spectacular sights in the solar system, a magnificent, glowing coma often visible even to the naked eye. Comet nuclei have been seen to produce sudden, bright flares and some even split into two, three, four, or five refions.

As the nucleus of a comet approaches the Sun, beginning at about the distance of the asteroid belt, its ices begin to vaporize and sublimate (change directly from ice to gas). This off-gassing releases gases of hydrogen, carbon, oxygen , nitrogen and other molecules, as well as dust particles. Streaming away at several hundred meters per second, they create an enormous coma hundreds of thousands of kilometers long, completely hiding the nucleus. The Sun's ultraviolet light electrically charges the gaseous molecules, ionizing and exciting them, causing them to fluoresce (emit light) much like a fluorescent light emits light following electrical stimulation. Microscopic mineral particles in the dust reflect and scatter the Sun's light. Only in 1970, during the first spacecraft observation of a comet, was a gigantic hydrogen cloud discovered surrounding the Coma. Depending on the size of the nucleus and its proximity to the Sun, this cloud can be much larger than the comet itself.

As the comet swings around the Sun on its elliptical orbit, gas and dust particles stream from the coma to create two types of tails: the gaseous ion tail, or Type I; and the dust tail, or Type II. In Type I, ionized gases form a thin, usually straight tail, sometimes millions of kilometers long. (The tail of the Great Comet of 1843 stretched out more than 136 million mi [220 million km].) In fact, the tails of comets are the largest measured entities in the solar system. The ion tail, glowing with incredible brightness, does not trail behind but is blown away from the head in a direction almost opposite the Sun by the "solar wind," a continual flow of magnetized plasma emitted by the Sun. The head collides with this plasma, which wraps around the nucleus, pulling the ionized particles with it. Depending on its position to the Sun, the tail may even be traveling almost ahead of the nucleus. A Type II tail is usually shorter and wider, and curves slightly because the heavier particles are carried away at a slower rate. The Great Comet of 1744 actually displayed six brilliant tails fanning above the horizon like peacock feathers.

Comet Hale-Bopp, which streamed across the skies in 1997, boasted a new feature: a third tail composed of electrically neutral sodium atoms. When completely observed using instruments with spectral filters that eliminated all but the yellow light emitted by fluorescing sodium atoms, the tail was more than 370,000 miles wide (600,000 km) and 31 million miles long (50 million km), streaming in a direction close but slightly different to that of the ion tail. Although the exact mechanism is not understood, the tail is thought to be formed of sodium atoms released by dust particles in the coma.

Comets may strike planets without leaving an impact crater . The atmospheric energy released by comet vaporization in the atmosphere can, however, be more powerful than a nuclear explosion. The Tunguska event in Siberia in 1908 is thought to have been the result of a comet or stony meteoroid explosion above the ground. In 1979, a United States Air Force space-test satellite took the first photograph of a comet colliding with the Sun. Late in 1994, the fragmented comet Shoemaker-Levy made spectacular serial collisions with Jupiter.

Some scientists argue that molecules released by comets' vaporized gases may have supplied important molecules in Earth's early atmosphere. When exposed to the Sun's radiation, these molecules began the formation of biochemical compounds that actually began the process of life on Earth—or gave a huge "jump-start" to the evolution of biomolcules. During the recent passage of Hale-Bopp, for example, scientists discovered a variety of complex organic chemicals in the comet.

Some aspects of this theory gained evidence from data gathered by the Polar spacecraft, launched by NASA in 1996. According to some interpretations of observations by the probe, comet-like objects up to 30–40 ft (9–12 m) in diameter may be hitting the atmosphere at the astounding rate of up to 43,000 per day. These cosmic snowballs usually disintegrate in the upper atmosphere, their content liquids and gases entering the weather cycle and eventually reaching the terrestrial surface as precipitation . Other scientists argue that the evidence of "snowballs" from space is an artifact of instrument background noise or interference.

In a pair of space missions planned for the early part of the twenty-first century, space probes will rendezvous with a pair of short-period comets, hopefully to help scientists reach a better understanding of the physics of comets. In 2004, NASA's Stardust mission plans to capture dust from the tail of Comet Wild 2, returning the samples to Earth for analysis. In 2011, the European Space Agency's Rosetta mission will rendezvous with Comet Wirtanen on its trip around the Sun. The Rosetta spacecraft will orbit the comet and send a probe to the surface.

See also Astronomy; Big Bang theory; Impact crater

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Comet

UXL Encyclopedia of Science
COPYRIGHT 2002 The Gale Group, Inc.

Comet

A comet—a Greek word meaning "long-haired"—is best described as a dirty snowball. It is a clump of rocky material, dust, and frozen methane, ammonia, and water that streaks across the sky on a long, elliptical (ovalshaped) orbit around the Sun. A comet consists of a dark, solid nucleus (core) surrounded by a gigantic, glowing mass (coma). Together, the core and coma make up the comet's head, seen as a glowing ball from which streams a long, luminous tail. The tail (which always points away from the Sun) is formed when a comet nears the Sun and melted particles and gases from the comet are swept back by the solar wind (electrically charged particles that flow out from the Sun). A tail can extend as much as 100 million miles (160 million kilometers) in length.

Age-old fascination

Through the ages, comets were commonly viewed as omens, both good and bad, because of their unusual shape and sudden appearance. A comet appearing in 44 b.c. shortly after Roman dictator Julius Caesar was murdered was thought to be his soul returning. A comet that appeared in 684 was blamed for an outbreak of the plague that killed thousands of people.

For centuries, many people believed Earth was at the center of the solar system, with the Sun and other planets orbiting around it. They also believed that comets were a part of Earth's atmosphere. In the sixteenth century, Polish astronomer Nicolaus Copernicus (1473–1543) proposed a theory that placed the Sun at the center of the solar system, with Earth and the other planets in orbit around it. Once astronomers finally determined that comets existed in space beyond Earth's atmosphere, they tried to determine the origin, formation, movement, shape of orbit, and meaning of comets.

Halley's comet

In 1687, English astronomer Edmond Halley (pronounced HAL-ee; 1656–1742) calculated the paths traveled by 24 comets. Among these, he found three—those of 1531, 1607, and 1682—with nearly identical paths. This discovery led him to conclude that comets follow an orbit around the Sun, and thus reappear periodically. Halley predicted that this same comet would return in 1758. Although he did not live to see it, his prediction was correct, and the comet was named Halley's comet. Usually appearing every 76 years, the comet passed by Earth in 1835, 1910, and 1986.

Words to Know

Astronomical unit (AU): Standard measure of distance to celestial objects, equal to the average distance from Earth to the Sun: 93 million miles (150 million kilometers).

Coma: Glowing cloud of mass surrounding the nucleus of a comet.

Ellipse: An oval or elongated circle.

Interstellar medium: Space between stars, consisting mainly of empty space with a very small concentration of gas atoms and tiny solid particles.

Nucleus: Core or center of a comet.

During its last pass over the planet, Halley's comet was explored by the European Space Agency probe Giotto. The probe came within 370 miles (596 kilometers) of Halley's center, capturing fascinating images of the 9-mile-long, 5-mile-wide (15-kilometer-long, 8-kilometer-wide) potato-shaped core marked by hills and valleys. Two bright jets of dust and gas, each 9 miles (15 kilometers) long, shot out of the core. Giotto 's instruments detected the presence of water, carbon, nitrogen, and sulfur molecules. It also found that the comet was losing about 30 tons of water and 5 tons of dust each hour. This means that although the comet will survive for hundreds more orbits, it will eventually disintegrate. Halley's comet will next pass by Earth in the year 2061.

Comet Hale-Bopp

On July 22, 1995, American astronomer Alan Hale and American amateur stargazer Thomas Bopp independently discovered a new comet just beyond the orbit of Jupiter. Considered by many astronomers to be one of the greatest comets of all time, Comet Hale-Bopp is immense. Its core is almost 25 miles (40 kilometers) in diameter, more than 10 times that of the average comet and 4 times that of Halley's comet. Hale-Bopp's closest pass to Earth occurred on March 22, 1997, when it was 122 million miles (196 million kilometers) away. Despite its great distance from Earth, the huge comet was visible to the naked eye for months before and after that date. Astronomers believed it was one of the longest times any comet had been visible. They estimate that Hale-Bopp will next visit the vicinity of Earth 3,000 years from now.

Nourishing snowballs

In mid-1997, scientists announced that small comets about 40 feet (12 meters) in diameter are entering Earth's atmosphere at a rate of about 43,000 a day. The discovery was made by the polar satellite launched by the National Aeronautics and Space Administration (NASA) in early 1996. American physicist Louis A. Frank, the principal scientist for the visible imaging system of the satellite, first proposed the existence of the bombarding comets in 1986.

These comets do not strike the surface of Earth because they break up at heights of 600 to 15,000 miles (960 to 24,000 kilometers) above ground. Sunlight then vaporizes the remaining small icy fragments into huge clouds. As winds disperse these clouds and they sink lower in the atmosphere, the water vapor contained within condenses and falls to the surface as rain. Scientists estimate that this cosmic rain adds one inch of water to Earth's surface every 10,000 to 20,000 years. Over the immense span of Earth's history (4.5 billion years), this amount of water could have been enough to fill the oceans.

Scientists also speculate that the simple organic chemicals (carbon-rich molecules) these comets contain might have fallen on Earth as it was first developing. They may have provided the groundwork for the development of the wide range of life on the planet.

The origin of comets

Comets are considered among the most primitive bodies in the solar system. They are probably debris from the formation of our sun and planets some 4.5 billion years ago. The most commonly accepted theory about where comets originate was suggested by Dutch astronomer Jan Oort in 1950. He believed that over 100 billion inactive comets lie at the frigid, outer edge of the solar system, somewhere between 50,000 and 150,000 astronomical units (AU) from the Sun. (One AU equals the distance from Earth to the Sun.) They remain there in an immense band, called the Oort cloud, until the gravity of a passing star jolts a comet into orbit around the Sun.

In 1951, another Dutch astronomer, Gerard Kuiper, suggested that there is a second reservoir of comets located just beyond the edge of our solar system, around 1,000 times closer to the Sun than the Oort cloud. His hypothetical Kuiper Belt was confirmed in 1992 when astronomers discovered the first small, icy object in a ring of icy debris orbiting the Sun. This ring is located between Neptune and Pluto (sometimes beyond Pluto, depending on its orbit), some 3.6 billion miles (5.8 billion kilometers) from Earth. Since 1992, astronomers have discovered more than 150 Kuiper Belt objects. Many of them are upwards of 60 miles (96 kilometers) in diameter. Several are much larger. In 2000, astronomers discovered one, which they call Varuna, that measures 560 miles (900 kilometers) in diameter, about one-third the size of the planet Pluto. Astronomers believe the ring is filled with hundreds of thousands of small, icy objects that are well-preserved remnants of the early solar system. They are interested in studying these objects because they want to know more about how Earth and the other major planets formed.

The death of comets

There are many theories as to what happens at the end of a comet's life. The most common is that the comet's nucleus splits or explodes, which may produce a meteor shower. It has also been proposed that comets eventually become inactive and end up as asteroids. One more theory states that gravity or some other disturbance causes a comet to exit the solar system and travel out into the interstellar medium.

[See alsoMeteors and Meteorites ]

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comet

The Columbia Encyclopedia, 6th ed.

Copyright The Columbia University Press

comet [Gr.,=longhaired], a small celestial body consisting mostly of dust and gases that moves in an elongated elliptical or nearly parabolic orbit around the sun or another star. Comets visible from the earth can be seen for periods ranging from a few days to several months. They were long regarded with awe and even terror and were often taken as omens of unfavorable events. In 1987 a comet orbiting the star Beta Pictoris was identified, and since then an increasing number of exocomets have been found.

The Orbits of Comets

Although the occurrence of many comets had been recorded, it was not until 1577 that the Danish astronomer Tycho Brahe suggested that they traveled in elongated rather than circular orbits. A century later Giovanni Borelli concluded that the orbits were parabolic and that comets passed through the solar system but once, never to return. In 1705, however, Edmond Halley concluded that the comet observed in 1682 was the same one that had been described in 1531 and 1607, and he predicted that it would return again in late 1758 or early 1759. The comet was sighted on Christmas Day in 1758, and it returned again in 1835, 1910, and 1986 (see Halley's comet). While some comets appear to have parabolic orbits (see parabola), others return to the inner solar system in highly elongated orbits with periods ranging from a hundred to thousands of years. Still others return at shorter intervals of less than 10 years and reach aphelion (the orbital point farthest from the sun) near the planet Jupiter; these have been captured into their smaller orbits by Jupiter's gravitational attraction.

Structure of Comets

A comet far from the sun consists of a dense solid body or conglomerate of bodies a few miles in diameter called the nucleus. As it approaches the sun the nucleus becomes enveloped by a luminous
"cloud"
of dust and gases called the coma; this luminosity is caused by the molecules absorbing and reflecting the radiation of the sun. According to the icy-conglomerate theory proposed by F. L. Whipple in 1949, the nucleus consists of frozen water and gases with particles of heavier substances interspersed throughout, thus being in effect a large, dirty snowball, although more recent research has suggested that comets may contain a higher proportion of dust and rock than previously proposed. The Stardust probe—passed near Comet Wild 2 in 2004, collected particles from the coma, and returned the samples to earth in 2006—found evidence that many of the dust particles were formed at high temperatures not found in the Oort cloud and Kuiper belt (see below), where comets are believed to have formed. Data from the Deep Impact mission, which sent a projectile crashing into Comet Tempel 1 in 2005, suggests that suggests that the interior structure of comets may consists of layers of accreted material. As the comet approaches the sun, the solar wind drives particles and gases from the near the surface of the nucleus and coma to form a tail which can extend as much as 100 million mi (160 million km) in length. Thus the tail always streams out in the direction opposite the sun; i.e., it follows the head as the comet approaches the sun and precedes it as the comet passes perihelion (its closest point to the sun) and moves away.

Near the sun a comet can change drastically in size and shape; it may even split into two or more pieces, as Comet Biela did in 1846, and Comet West did in 1976. The comas of comets vary widely in size, some being the size of the earth or larger. However, the nucleus, which makes up virtually all a comet's mass, is small; in 1986 the Giotto and Vega spacecrafts observed Comet Halley's nucleus to be only about 6 mi (10 km) in diameter. In 2014 Rosetta became the first space probe to orbit a comet's nucleus (that of Comet 67P); it also deployed a lander on the comet. Comets lose material and thus brightness with successive passages near the sun. Some of this material moves around the comet's orbit as a stream of meteoroids (see meteor); when the earth passes through this path, a meteor shower is observed.

In 1992 the periodic comet Shoemaker Levy 9 made an extremely close passage of Jupiter. The tidal stresses induced by the giant planet's gravity shattered the comet's nucleus, estimated to have been 5–9 km (3–5 mi) in diameter, into more than 20 major fragments, the largest of which was about 4 km (2.5 mi) in diameter. Two years later, the returning fragmented comet crashed into Jupiter; observations from both terrestrial observatories and artificial satellites such as the Hubble Space Telescope yielded vast amounts of information about the structure of comets and about Jupiter's atmosphere.

In 1996 the Polar satellite discovered a constant rain of small comets impacting the earth. Unlike large comets, whose cores are estimated to be as much as 25 mi (40 km) in diameter, these are only up to 40 ft (12 m) wide. It is estimated that as many as 43,000 reach the earth each day and break up at altitudes of 600–15,000 mi (950–24,000 km). Also in 1996 the ROSAT satellite (see X-ray astronomy) detected X-rays emanating from the Comet Hyakutake. This was completely unexpected, and can be explained by no known mechanism. Observation of more large comets passing through the solar system by orbiting X-ray observatories will be necessary to corroborate this finding.

The Origin of Comets in the Solar System

The Oort Cloud

The origin of the solar system's comets is still uncertain. They were once thought to have originated outside the solar system, but more recent theories suggest they were formed during the formation of the solar system and are permanent members of it. According to the storage-cloud hypothesis proposed by J. H. Oort and since modified, a spherical shell of more than 100 billion comets surrounds the solar system at a distance of 20,000 AU to 50,000 AU or greater (1 AU, or astronomical unit, being the mean distance from the earth to the sun); some astronomers have suggested an inner Oort cloud exists beginning at 2,000 to 5,000 AU and extending to 20,000 AU. While the comets move very slowly in this huge storage cloud, a passing star may change the orbits of bodies in the outer reaches of the Oort cloud enough to force some of them into the inner part of the solar system. The mechanism for the Oort cloud's creation, however, is unclear; it has been suggested that the Oort cloud may include a significant amount of material that originated outside the solar system and was gravitationally captured by the sun.

The Kuiper Belt

In 1951, G. P. Kuiper, noting that Oort's cloud of comets did not adequately account for the population of short-period comets (those making complete orbits around the sun in less than 200 years), proposed the existence of a disk-shaped region of minor planets outside the orbit of Neptune, now called the Kuiper belt, as a source for such comets. The Kuiper belt acts as a reservoir for these in the same way that the Oort cloud acts as a reservoir for the long-period comets. This theory was validated in 1992 with the discovery of the first of the more than 70,000 so-called transneptunian objects, bodies more than 60 mi (100 km) in diameter in an orbit 30–50 AU from the sun. Astronomers regard Pluto not as a planet but rather as dwarf planet that is a member of the Kuiper belt. The discoveries of several Kuiper belt objects led to this view. Eris, an object discovered in 2003 (and originally nicknamed Xena), has an elongated orbit that extends to roughly three times the distance of Pluto's, has a diameter (1,500 mi/2,400 km) slightly larger than that of Pluto, and has a moon; Quaoar is more than half the size of Pluto; and Ixion and Varuna are almost half the size of Pluto. 2003 VS2 (roughly a fourth the size of Pluto) and a number of other Kuiper belt objects, called plutinos, have an orbital synchrony with Neptune like that of Pluto (Neptune completes three orbits around the sun in the same time that Pluto and the plutinos complete two orbits).

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Comets

Space Sciences
COPYRIGHT 2002 The Gale Group Inc.

Comets

A bright comet is a spectacular astronomical event. Throughout history, comets have left a strong impression on those who have witnessed their appearances. The name comes from the Greek kometes, meaning "the long-haired one." Ancient Greeks thought comets to be atmospheric phenomena, part of the "imperfect" changeable Earth, not of the "perfect" immutable heavens. Today we know they are "icy conglomerates," as proposed in 1950 by Fred Whipple—that is, chunks of ice and dust left over from the formation of the solar system some 4.6 billion years ago.

Comets are among the most primitive bodies in the solar system. Because of their orbits and small sizes, comets have undergone relatively little processing, unlike larger bodies, such as the Moon and Earth, which have been modified considerably since they formed. The chemical composition of comets contains a wealth of information about their origin and evolution as well as the origin and evolution of the solar system. Hence, comets are often called cosmic fossils.

When a comet is far from the Sun, it is an inert icy body. As it approaches the Sun, heat causes ices in the nucleus to sublimate , creating a cloud of gas and dust known as the coma. Sunlight and solar wind will push the coma gas and dust away from the Sun creating two tails. The dust tail is generally curved and appears yellowish because the dust particles are scattering sunlight. The gas (or ion) tail is generally straight and it appears blue
because its light is dominated by emission from carbon monoxide ions. The appearance of comets in photographs can give the erroneous impression that they streak through the night sky like a meteor or a shooting star. In fact, comets move slowly from night to night with respect to the stars and can sometimes be visible for many weeks, as was the case with comet Hale-Bopp in 1997 and with comet Halley during its 1985-1986 appearance.

Comet Halley is not the brightest comet, but it is the most famous, mainly because it is the brightest of the predictable comets. It was named after Edmund Halley, an eighteenth-century British astronomer who was the first to calculate the orbits of comets. Comet Halley's orbit has an average period of seventy-six years. Its closest approach to the Sun (perihelion) is between the orbits of Venus and Mercury (0.59 astronomical units ), and its aphelion (farthest distance from the Sun) is at 35 AU, beyond Neptune's orbit. The orbit has an inclination of 162 degrees with respect to the ecliptic . This means that comet Halley orbits the Sun clockwise when seen from the north, whereas Earth orbits the Sun counterclockwise.

The study of comets is a very active field of science. In 1986 a flotilla of spacecraft were used to study comet Halley. In the first decade of the twenty-first century, several spacecraft are scheduled to be launched to encounter and study a number of comets. In addition to space-based studies, ground-based observations of comets have yielded a wealth of information.

The Comet's Nucleus

All of the activity in a comet originates in its nucleus, which is composed of roughly equal amounts of ices and dust. Water ice is the most abundant of the ices, comprising about 80 percent of the total. So far, only the nuclei of comets Halley and Borrelly have been imaged in detail. Comet Halley turned out to be larger, darker, and less spherical than expected by most astronomers. The images of comet Borrelly's nucleus obtained in September 2001 by NASA's Deep Space 1 spacecraft show considerable similarity with those of comet Halley. Halley's nucleus is peanut-shaped, approximately 18 kilometers (11 miles) long and 8 kilometers (5 miles) wide. The reflectivity (or albedo) is 4 percent, which is as dark as coal. The size, albedo, and approximate shape of several other cometary nuclei have been determined. Comet Halley's nucleus seems to be typical among comets with relatively short orbital periods, and there are much larger nuclei such as that of comet Hale-Bopp. So far, the cometary nuclei studied in detail appear to have most of their surface covered by an inert mantle or crust. The active (exposed ice) fraction of their surface is small; in comet Halley, this fraction is somewhere between 15 and 30 percent.

The development of a crust can suppress the activity of cometary nuclei and give them an asteroidal appearance. The best example to date is comet Wilson-Harrington, which was discovered in 1949 and was lost until it was rediscovered as an inert object and given the asteroid number 4015. The behavior of this object has added credence to the long-held expectation that some Earth-crossing asteroids are extinct or dormant comet nuclei.

The Composition of Comets

The composition of cometary nuclei is primarily inferred from studies of the coma components, namely gas, plasma (ions), and dust. So far, twentyfour
different molecules have been identified in comets, ten of which were discovered in comet Hale-Bopp. The molecules observed in comets and their relative abundances are very similar to those observed in dense interstellar molecular cloud cores, which is the environment where star formation occurs. Thus, it appears that comets underwent little processing in the solar nebula and they preserve a good record of its original composition.

Information on the composition of cometary dust particles was scarce before 1986. Studies of the dust in comet Halley and other comets confirmed that some of the grains are silicates, more specifically crystalline olivine (Mg, Fe)2 SiO4 and pyroxene (Mg, Fe, Ca) SiO3. Another major component of the dust in comet Halley was organic dust. These small solid particles were discovered by the visiting spacecraft and were called "CHON" because they were composed almost exclusively of the elements carbon, hydrogen, oxygen, and nitrogen.

The Origins of Comets

Dutch astronomer Jan Hendrik Oort noted in 1950 that the source of new comets was a shell located between 20,000 and 100,000 AU from the Sun. The existence of the Oort cloud is now widely accepted. Astronomers believe that comets in the Oort cloud formed near Uranus and Neptune and were gravitationally scattered by these two planets into their current location. In addition to the Oort cloud, there is another reservoir that was proposed in 1951 by Dutch-born American astronomer Gerard Peter Kuiper as a ring of icy bodies beyond Pluto's orbit. This Kuiper belt is considered to be the main source of Jupiter-family comets, which are those with low-inclination and short-period orbits.

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Comet Capture

Space Sciences
COPYRIGHT 2002 The Gale Group Inc.

Comet Capture

Comets are the most volatile -rich minor bodies in the solar system. It has been suggested that impacts with comets and asteroids provided Earth with much of its water. Although most comets are less accessible than near-Earthasteroids , their high water content makes them an economically attractive resource for space mining. The possibility that some near-Earth asteroids are extinct or dormant cometary nuclei means that this water-rich resource may be more accessible than was once thought.

Recent spacecraftand ground-based studies of comets have confirmed and refined Whipple's "dirty snowball" model for cometary nuclei. Cometary material is composed principally of water ice and other ices (including CO, CO2, CH4, C2 H6, and CH3 OH) mixed with cosmic dust grains. The passages of most Oort cloud comets through the inner solar system are not predictable. In addition, the highly elongated and inclined trajectories of these comets make them difficult targets with which to match orbits. In contrast, Jupiter-family comets tend to have predictable, well-determined orbits with short periods and low inclinations. Therefore, a future mining mission would most likely target a Jupiter-family comet.

The capture of an active comet as a source of water and other volatile elements is a difficult proposition. In the vicinity of Earth the jet-like gas that flows from a comet's nucleus would have a stronger influence on its trajectory than any force humans could apply to the comet. This behavior would make transporting an active comet into a suitable near-Earth orbit, and maintaining it there, very unlikely. The Earth-impact hazard posed by a sizable comet* or comet fragment in an unstable near-Earth orbit would be unacceptable. For example, even if the trajectory of a cometary fragment could be manipulated to produce capture into a high-Earth orbit, bringing the material down to low-Earth orbit (e.g., to the space station) would be difficult. The Moon's gravitational pull would make the trajectory extremely difficult to predict and control.

Capture into a lunar orbit would also be problematical. Lunar orbits tend to be unstable because of gravitational influences from Earth and the Sun. Another difficulty that must be resolved is the current uncertainty about the consistency of cometary nuclei. Not only is the bulk density of cometary nuclei unknown (estimates range from 0.3 g/cm3 to greater than 1 g/cm3;
liquid water has a density of 1 g/cm
3), we do not know the cohesiveness of this material. Such uncertainties make it impossible to predict the mechanical properties of cometary material and the way a comet nucleus would react to a "nudge" to change its trajectory. A comet nucleus may or may not behave as a rigid object does; it might instead break up into fragments when a force is applied to change its orbit.

A more attractive approach to harvesting cometary material would be to send a robotic spacecraft to mine the comet. Returning fine-grained material and/or liquid water to Earth orbit would greatly lower the risks. A cargo spacecraft would be easier to control than a comet fragment, and even if an uncontrolled atmospheric entry occurred, the water and/or fine-grained material would vaporize or rain down harmlessly onto Earth's surface.

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Comets

Encyclopedia of Occultism and Parapsychology
COPYRIGHT 2001 The Gale Group Inc.

Comets

Throughout human history comets have been regarded as auguries of disasters such as famine, plague, or war. The most recent outbreak of widespread concern that a comet might portend disaster occurred in 1973 when the comet Kohoutek was announced. For the first time in more than a generation, there arose the possibility that a bright comet, plainly visible with the naked eye, would be seen by the majority of people. A variety of speculations on the spiritual and prophetic implications of the comet were made, but the comet did not prove to be as spectacular as hoped, and none of the predicted changes signaled by its appearance occurred. No such speculation seems to have occurred at the time of the return of Halley's Comet in 1986.

In the past century comets have also figured in speculations about the history of the earth. In Ragnarok: the Age of Fire and Gravel (1883), Ignatius Donnelly assembled legends and religious beliefs tending to show that the earth was affected by a collision with a comet that created the Pleistocene Ice Age. In the 1950s, Immanuel Velikovsky connected the theme of a comet disaster with biblical prophecy in his book Worlds in Collision.

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comet

comet Small, icy Solar System body in orbit around the Sun. The solid nucleus of a comet is small, that of Halley's comet measures just 16 × 8km (10 × 5mi), and comprises rock and dust particles embedded in ice. As the comet approaches the Sun, evaporation begins, and jets of gas and dust form the luminous coma. Later, radiation pressure from the Sun and solar wind may send dust and gas streaming away as a tail, as much as 150 million km in length. There are three main types of comet: short-period comets often have their aphelia at approximately the distance of Jupiter's orbit. Long-period comets have aphelia near or beyond Neptune's orbit. The third type are either called non-periodic, or hyperbolic. Their periods may be as much as several million yaers. See also meteor

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comet

comet A small body composed of meteoric dust and frozen ices (H2O, CO2, CO, HCHO) in a highly elliptical or parabolic orbit around the Sun. The average perihelion distance is less than 1 AU, and the average aphelion distance is about 104AU. Comets are derived from the Oort cloud and have average lifetimes of about 100 passages. Comet nucleii are irregular in shape, a few kilometres in diameter and have a low density (100–400 kg/m3). Due to solar radiation, they emit gas and dust, forming the characteristic tail, when within a few astronomical units of the Sun; the dust composition appears to resemble that of primitive carbonaceous chondrites.

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Comet-Planet Collisions

Space Sciences
COPYRIGHT 2002 The Gale Group Inc.

COMET-PLANET COLLISIONS

Many comets are in Earth-crossing orbits and collisions do occur between comets and planets. A spectacular example of such a collision was the impact of comet Shoemaker-Levy 9 with Jupiter in July 1994. It is now well established that an impact with an asteroid or comet created a large crater at the edge of the Yucatan Peninsula 65 million years ago. Known as the Chicxulub impact, this event almost certainly caused the extinction of the dinosaurs. Efforts are underway to study the population of potential hazards from both comets and asteroids in sufficient detail to predict and prevent future large impacts.

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comet

com·et
/ ˈkämit/
•
n.
a celestial object consisting of a nucleus of ice and dust and, when near the sun, a “tail” of gas and dust particles pointing away from the sun.
DERIVATIVES:com·et·ar·y
/ ˈkämiˌterē/ adj.

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